Optical semiconductor device and manufacturing method therefor

ABSTRACT

A manufacturing method for an optical semiconductor device, including disposing a semiconductor element that has a polarization dependent gain or polarization dependent loss between optical waveguide modes differing in the direction of polarization, positioning a lens at one end face side of the semiconductor element based on an optical coupling loss between the lens and the semiconductor element, and repositioning the lens based on the polarization dependent gain or the polarization dependent loss of the semiconductor element.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 12/498,425 filed Jul. 7, 2009, and is based upon and claims thebenefits of priority from Japanese Patent Application No. 2008-180554filed on Jul. 10, 2008, the entire contents of which being incorporatedherein by reference.

FIELD

The present invention relates to an optical semiconductor device and amanufacturing method therefor.

BACKGROUND

With the recent dramatic increase in demand in communications,high-capacity, high-speed photonic network applications have spread tothe extent that even metro access systems are accessible to subscribers.

An optical module provided with a semiconductor optical amplifier (SOA)(SOA module) has a dramatically simplified structure compared to thecurrently popular optical module provided with an optical fiberamplifier (optical fiber amplifier module). The optical module (SOAmodule) allows for greater miniaturization and price reduction, andapplication to the next-generation access network and datacommunications is under consideration.

An SOA module may be equipped with, for example, an input-side opticalfiber, an input-side lens system, an SOA chip, an output-side lenssystem, and an output-side optical fiber. Signal light is input to theinput-side optical fiber, optically coupled to the SOA chip through theinput-side lens system, amplified in the SOA chip, optically coupled tothe output-side optical fiber through the output-side lens system, andthen output from the output-side optical fiber.

In an optical network, the signal light input to the SOA module is notalways polarized in the same way, and the polarization state varies withtime.

Therefore, for the SOA module having input signal light with a constantintensity to always output signal light with a constant intensity, theSOA module needs to have an optical gain that does not vary with thepolarization of the input signal light.

SUMMARY

According to an aspect of the invention, a manufacturing method for anoptical semiconductor device, including disposing a semiconductorelement that has a polarization dependent gain (PDG) or polarizationdependent loss (PDL) between optical waveguide modes differing in thedirection of polarization, positioning a lens at one end face side ofthe semiconductor element based on an optical coupling loss between thelens and the semiconductor element, and repositioning the lens based onthe polarization dependent gain or the polarization dependent loss ofthe semiconductor element.

According to an another aspect of the invention, an opticalsemiconductor device, includes a semiconductor element that providesdifferent spot sizes on an element end face for optical waveguide modesdiffering in the direction of polarization, and a lens positioned at oneend face side of the semiconductor element, wherein a spot size at abeam waist position of the lens is smaller than a spot size in either ofthe optical waveguide modes.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an opticalsemiconductor device (SOA module) according to a first embodiment of thepresent invention;

FIG. 2 is a diagram illustrating positional adjustment based on theoptical power in a manufacturing method for the optical semiconductordevice (SOA module) according to the first embodiment of the presentinvention;

FIGS. 3A and 3B are diagrams illustrating compensation for a PDG of anSOA chip in the manufacturing method for the optical semiconductordevice (SOA module) according to the first embodiment of the presentinvention;

FIGS. 4A and 4B are diagrams illustrating a relationship between thedisplacement in an optical axis direction and the inter-polarizationdifference in optical coupling loss in positional adjustment based onthe PDG of the SOA chip in the manufacturing method for the opticalsemiconductor device (SOA module) according to the first embodiment ofthe present invention;

FIGS. 5A and 5B are diagrams illustrating the manufacturing method forthe optical semiconductor device (SOA module) according to the firstembodiment of the present invention;

FIG. 6 is a flowchart illustrating processes of the manufacturing methodfor the optical semiconductor device (SOA module) according to the firstembodiment of the present invention;

FIGS. 7A to 7C are diagrams illustrating a manufacturing method for anoptical semiconductor device (SOA module) according to a secondembodiment of the present invention;

FIG. 8 is a flowchart illustrating processes of the manufacturing methodfor the optical semiconductor device (SOA module) according to thesecond embodiment of the present invention;

FIGS. 9A and 9B are diagrams illustrating a lens jig used in themanufacturing methods for the optical semiconductor device (SOA module)according to the embodiments of the present invention;

FIG. 10 is a diagram illustrating another configuration of asemiconductor element (SOA chip) of the optical semiconductor device(SOA module) according to the embodiments of the present invention;

FIG. 11 is a diagram illustrating another configuration of the opticalsemiconductor device (SOA module) according to the embodiments of thepresent invention;

FIG. 12 is a diagram illustrating another configuration of the opticalsemiconductor device (SOA module) according to the embodiments of thepresent invention;

FIG. 13 is a diagram illustrating another configuration of the opticalsemiconductor device (SOA module) according to the embodiments of thepresent invention;

FIG. 14 is a diagram illustrating another configuration of thesemiconductor element of the optical semiconductor device (SOA module)according to the embodiments of the present invention; and

FIG. 15 is a diagram illustrating an optical receiver including theoptical semiconductor device (SOA module) according to the embodimentsof the present invention and an optical communication system includingthe optical receiver.

DESCRIPTION OF EMBODIMENTS

An SOA module that has a polarization-independent optical gain isprovided.

If the polarization dependent gain (PDG) of the SOA chip in an SOAmodule is reduced, the SOA module may have a polarization-independentoptical gain.

However, in many cases, because of variations in crystal growthconditions and process conditions during manufacture of SOA chips, thePDG of the SOA chips, which may have a maximum magnitude of several dB,varies with the wafer or within a wafer by several dB.

Such variations of the PDG of the SOA chips are hard to control, andtherefore, it is difficult to stably manufacture SOA chips having a lowPDG. Accordingly, it is also difficult to stably manufacture SOA modulesthat achieve a low PDG, and thus, it is difficult to stably provide SOAmodules that achieve a polarization-independent optical gain.

The problem concerns not only the SOA module having the SOA chipdescribed above but also an optical module that includes a semiconductorelement having a PDG or polarization dependent loss (PDL) that varieswith the semiconductor element and needs to have stablepolarization-independent characteristics.

In the following, optical semiconductor devices and manufacturingmethods therefor according to embodiments of the present invention willbe described with reference to the drawings.

First Embodiment

First, an optical semiconductor device and a manufacturing methodtherefor according to a first embodiment will be described withreference FIGS. 1 to 6.

In this embodiment, as a manufacturing method for an opticalsemiconductor device, a manufacturing method for a semiconductor opticalamplifier (SOA) module (an optical module) will be described, forexample.

As illustrated in FIG. 1, an SOA module 1 has a module housing 2, an SOAchip 3 disposed in the module housing 2, an input-side lens system 4disposed on the input side of the SOA chip 3, an input-side opticalfiber 5 optically coupled to an input port of the SOA chip 3 by theinput-side lens system 4, an output-side lens system 6 disposed on theoutput side of the SOA chip 3, and an output-side optical fiber 7optically coupled to an output port of the SOA chip 3 by the output-sidelens system 6. In FIG. 1, reference numeral 8 denotes a stage.

The SOA chip (semiconductor device, optical device) 3 has a port towhich signal light is input at an input end face of an optical waveguideand a port from which signal light is output at an output end face ofthe optical waveguide. The SOA chip 3 amplifies the intensity of theinput signal light by stimulated emission that is generated in an activelayer in the chip.

In the manufacturing method for the SOA module according to thisembodiment, the SOA module 1 is manufactured as described below.

First, the SOA chip 3, the input-side lens system (one lens system) 4,the output-side lens system (the other lens system) 6, the input-sideoptical fiber 5 (one optical fiber), and the output-side optical fiber 7(the other optical fiber) are provided (see FIG. 1).

An SOA chip 3 is provided that differs in spot size on the chip end face(element end face) between optical waveguide modes differing in thedirection of polarization (see FIG. 3A). In this embodiment,polarization is represented as two orthogonal polarization states: thetransverse electric (TE) mode and the transverse magnetic (TM) mode.That is, an SOA chip that has a polarization dependent gain in theoptical waveguide modes differing in the direction of polarization isprovided.

An output side lens system 6 is configured so that a spot size at a beamwaist position smaller than the spot size on the end face of the SOAchip 3 for respective optical waveguide modes (the TE mode and the TMmode in this embodiment) (see FIG. 3A). In this embodiment, the outputside lens system 6 includes a first lens 6A and a second lens 6B.

The input-side lens system 4 may be a lens system capable of opticallycoupling signal light input through the input-side optical fiber 5 tothe SOA chip 3 to reduce the optical coupling loss. In this embodiment,the input side lens system 4 includes a third lens 4A and a fourth lens4B.

The input-side lens system 4 and the input-side optical fiber 5 aredisposed on the side of the input end face of the SOA chip 3, and theoutput-side lens system 6 and the output-side optical fiber 7 aredisposed on the output side end face of the SOA chip 3 (see FIG. 1).

In this embodiment, in order from the closest to the SOA chip 3 to thefurthest away from the SOA chip 3, the first lens 6A is disposed at theside of the output end face of the SOA chip 3 closest to the SOA chip 3,the second lens 6B is disposed next to the first lens 6A, and theoutput-side optical fiber 7 is disposed next to the second lens 6B andfurthest away from the SOA chip 3. On the side of the input end face ofthe SOA chip 3, in order from closest to the SOA chip 3 to furthest awayfrom the SOA chip 3, the third lens 4A is disposed next to the SOA chip3, the fourth lens 4B is disposed next to the third lens 4A, and theinput-side optical fiber 5 is disposed next to the fourth lens 4B andfurthest away from the SOA chip 3 (see FIG. 1).

As illustrated in FIG. 2, an optical power meter 9 is connected to boththe input-side optical fiber 5 and the output-side optical fiber 7. Thepositions of the input-side lens system 4, the output-side lens system6, the input-side optical fiber 5, and the output-side optical fiber 7are adjusted to increase the power of the light coupled from the endfaces of the SOA chip 3 to the input-side optical fiber 5 and theoutput-side optical fiber 7 via the input-side lens system 4 and theoutput-side lens system 6, respectively (that is, to reduce the opticalcoupling loss). In FIG. 2, for convenience of explanation, only theoutput side is illustrated, and illustration of the input side isomitted.

In this embodiment, as illustrated in FIG. 2, by applying a current tothe SOA chip 3 to make the SOA chip 3 emit amplified spontaneousemission (ASE) light from both end faces, the positions of the firstlens 6A, the second lens 6B, and the output-side optical fiber 7 areadjusted to increase the power of the light coupled to the output-sideoptical fiber 7 via the first lens 6A and the second lens 6B in theoutput-side lens system 6.

Similarly, the positions of the input-side lens system 4 and theinput-side optical fiber 5 are adjusted to increase the power of thelight coupled to the input-side optical fiber 5 via the third lens 4Aand the fourth lens 4B in the input-side lens system 4.

After the positions are adjusted, the components other than the firstlens 6A, the input-side lens system 4, the input-side optical fiber 5,the second lens 6B, and the optical-side optical fiber 7, are fixed (seeFIG. 1).

The PDG of the SOA chip 3 (which is an indicator of the polarizationdependency of the optical gain of the SOA chip 3) is defined as thedifference between the maximum gain and the minimum gain of the SOA chip3 for input signal light in various polarization states. In general, forthe SOA chip 3, one of the two orthogonal linear polarization modes TEand TM is associated with the maximum PDG, and the other is associatedwith the minimum PDG. Thus, the PDG is expressed as PDG=|(TE gain)−(TMgain)|.

Furthermore, the PDG of the SOA module 1 is determined by totaling thePDG of the SOA chip 3 installed in the SOA module 1 and theinter-polarization difference in optical coupling loss between the SOAchip 3 and the input-side optical fiber 5 or the output-side opticalfiber 7 (which is an indicator of the polarization dependency of theoptical coupling loss between the SOA chip 3 and the optical fiber 5 and7).

In general, the PDG of the SOA chip 3 has a greater influence. Andbecause of variations in crystal growth conditions and processconditions during manufacture of SOA chips, the PDG of SOA chips, whichmay have a maximum magnitude of several dB, varies between wafers orwithin a wafer by several dB.

Thus, in this embodiment, after the positions are adjusted based on theoptical power as described above, positional adjustment is carried outby displacing the output-side lens system 6 (the first lens 6A in thisembodiment) in an optical axis direction so that the inter-polarizationdifference in optical coupling loss between the SOA chip 3 and theoutput-side optical fiber 7 is suitable for compensation for the PDG ofthe SOA chip 3 (see FIG. 3A).

That is, in this embodiment, in the optical coupling adjustment process(lens system adjustment process) during manufacture of the SOA module,the first lens 6A that has been positioned to reduce the opticalcoupling loss as described above is intentionally shifted in the opticalaxis direction (see FIG. 3A).

For example, even if the focal position of the output-side lens system 6has been adjusted to be adjusted to the position of the output end faceof the SOA chip 3 by the first positional adjustment of the output-sidelens system 6 described above (see the position depicted by the solidline in FIG. 3A), the focal position of the output-side lens system 6shifts from the position of the output end face of the SOA chip 3 due tothe second positional adjustment of the output-side lens system 6 (seethe position depicted by the broken line in FIG. 3A).

As a result, an optical coupling loss may occur in the SOA module 1.However, the inter-polarization difference in optical coupling lossbetween the SOA chip 3 and the output-side optical fiber 7 can beadjusted.

After the positional adjustment based on the PDG of the SOA chip 3 iscarried out, the first lens 6A is fixed (see FIG. 1).

In this way, a polarization-independent SOA module that provides apolarization-independent optical gain, which is one of the opticalamplification characteristics of the part of the SOA module 1 betweenthe input-side optical fiber 5 and the output-side optical fiber 7, canbe manufactured.

Next, a principle of compensating for a variation of the PDG of the SOAchip 3 by adjusting the inter-polarization difference in opticalcoupling loss between the SOA chip 3 and the output-side optical fiber 7will be described.

In this embodiment, as described above, an SOA chip that differs in spotsize on the chip end face between the TE mode and the TM mode is used asthe SOA chip 3. The SOA chip also has different gains in the opticalwaveguide modes differing in the direction of polarization. Furthermore,a lens system that is configured to provide a spot size at a beam waistposition smaller than the spot size on the end face of the SOA chip 3 ineither the TE mode or the TM mode is used as the output-side lens system6 (see FIG. 3A).

Therefore, when the focal position of the lens system 6, which is theposition of the waist of the beam propagating through the lens system 6,is adjusted to the position of the end face of the SOA chip 3 (that is,when the distance between the lens 6A in the lens system 6 that isdisposed at a position closest to a waveguide end face 3X of the SOAchip 3 and the waveguide end face 3X of the SOA chip 3 is equal to thelens focal length), on the waveguide end face 3X of the SOA chip 3, thespot size W_(lens) of the beam propagating through the lens system 6(spot size for the lens system 6), the spot size W_(TE) of the beamguided through an SOA optical waveguide 3A in the TE waveguide mode, andthe spot size W_(TM) of the beam guided through the SOA opticalwaveguide 3A in the TM waveguide mode conform to the followingrelationship: W_(lens)<W_(TE)<W_(TM).

When an SOA chip and an optical fiber are optically coupled to eachother by a lens system, the optical coupling loss between the SOA chipand the optical fiber is proportional to the mode overlapping integralη_(TE) for the propagation mode and the TE waveguide mode of the lenssystem on the end face of the SOA chip, or the mode overlapping integralη_(TM) for the propagation mode and the TM waveguide mode of the lenssystem.

Thus, the optical coupling loss between the SOA chip and the opticalfiber can be reduced by making the spot size of the lens system in thepropagation mode and the spot size of the SOA chip in the TE waveguidemode and the TM waveguide mode close to or equal to each other.

For example, when the spot size of the beam guided through the SOAoptical waveguide of the SOA chip differs between the TE waveguide modeand the TM waveguide mode, it can be assumed that the lens system isdesigned so that the spot size W_(lens) of the lens system isapproximately equal to the larger one of the spot size W_(TE) for the TEwaveguide mode or the spot size W_(TM) for the TM waveguide mode (forexample, W_(lens)≈W_(TE)>W_(TM)).

Typically, since the focal position of the lens system is adjusted tothe position of the end face of the SOA chip to reduce the opticalcoupling loss, the spot size W_(lens) of the lens system on the end faceof the SOA chip is or is close to the spot size at the beam waistposition (the minimum spot size).

In this case, even if the lens system is displaced in the optical axisdirection to adjust the distance of the lens system from the end face ofthe SOA chip, the size of the condensed light spot (the diameter of thebeam condensed by the lens) cannot be smaller than the spot size at thebeam waist position. Therefore, the relationship between the modeoverlapping integrals η_(TE) and η_(TM) cannot be changed, and thus, theinter-polarization difference in optical coupling loss between the SOAchip and the optical fiber cannot be adjusted.

However, according to this embodiment, since the relationshipW_(lens)<W_(TE)<W_(TM) is met on the waveguide end face 3X when theposition of the waist of the beam propagating through the lens system 6is adjusted to the position of the end face of the SOA chip 3 (asdepicted by the solid line in FIG. 3A), the inter-polarizationdifference in optical coupling loss between the SOA chip 3 and theoptical fiber 7 can be adjusted by adjusting the distance of the lenssystem 6 from the waveguide end face 3X of the SOA chip 3 by displacingthe first lens 6A in this embodiment in the optical axis direction asdepicted by the broken line in FIG. 3A.

That is, although the spot sizes W_(TE) and W_(TM) of the SOA chip 3 inthe TE mode and the TM mode are kept constant at the position of the endface of the SOA chip 3, the spot size W_(lens) of the lens system 6 canbe adjusted only by being increased (as depicted by the broken line inFIG. 3A). This is because the spot size of the light condensed by thelens system 6 increases when defocusing of the lens system 6 withrespect to the end face of the SOA chip 3 is achieved by adjusting thedistance of the first lens 6A in this embodiment from the end face ofthe SOA chip 3.

FIG. 3B is a graph of a relationship between the displacement of thelens system 6 from the focal position in the optical axis direction andthe spot size of the beam from the optical fiber 7 condensed on thewaveguide end face 3X of the SOA chip 3.

In FIG. 3B, an SOA chip 3 is used that has spot sizes (mode diameters)of substantially 3.0 μm and 4.0 μm on the waveguide end face 3X of theSOA chip 3 for the TE mode and the TM mode, respectively (the spot sizeis the diameter because the spot is substantially circular in thisembodiment). In addition, in FIG. 3B, a lens system 6 is used that has aspot size of substantially 2.0 μm for the condensed beam (signal light)from the optical fiber 7 at the beam waist position.

As illustrated in FIG. 3B, as the displacement of the first lens 6A fromthe focal position in the optical axis direction increases, the spotsize of the beam from the optical fiber 7 condensed on the waveguide endface 3X of the SOA chip 3 increases.

Therefore, the relationship W_(lens)<W_(TE)<W_(TM) between the spotsizes on the waveguide end face 3X of the SOA chip 3 at the time whenthe beam waist position of the beam propagating through the lens system6 is adjusted to the position of the end face of the SOA chip 3 can bechanged into a relationship W_(TE)<W_(lens)<W_(TM) and further into arelationship W_(TE)<W_(TM)<W_(lens) by adjusting the distance of thelens system 6 (the first lens 6A in this embodiment) from the waveguideend face 3X of the SOA chip 3, as illustrated in FIGS. 3A and 3B.

Accordingly, the relationship η_(TE)>η_(TM) of the overlapping integralsη_(TE) and η_(TM) between the TE waveguide mode of the SOA chip 3 andthe propagation mode of the lens system 6, and between the TM waveguidemode of the SOA chip 3 and the propagation mode of the lens system 6 (inthe case where the beam waist position of the beam propagating throughthe lens system 6 lies on the end face of the SOA chip 3) changes into arelationship η_(TE)=η_(TM) or into a relationship η_(TE)<η_(TM).

As described above, by adjusting the distance of the lens system (thefirst lens 6A in this embodiment) from the waveguide end face 3X of theSOA chip 3, the relationship between the mode overlapping integralsη_(TE) and η_(TM) may be changed. That is, by adjusting the distance ofthe lens system (the first lens 6A in this embodiment) from thewaveguide end face 3X of the SOA chip 3, a state where the opticalcoupling loss for the TE mode is greater than the optical coupling lossfor the TM mode may be changed into a state where the optical couplingloss for the TE mode is smaller than the optical coupling loss for theTM mode. Therefore, the inter-polarization difference in opticalcoupling loss between the SOA chip 3 and the optical fiber 7 can beadjusted.

The above states can be achieved only by making the spot size W_(lens)of the lens system 6 on the end face of the SOA chip 3 smaller than thespot sizes W_(TE) and W_(TM) of the TE mode and the TM mode when thebeam waist position of the beam propagating through the lens system 6 isadjusted to the position of the waveguide end face 3X of the SOA chip 3as described above.

However, if the lens system is designed to satisfy the relationshipW_(lens)≈W_(TE)>W_(TM) on the end face of the SOA chip when the beamwaist position of the beam propagating through the lens system isadjusted to the position of the end face of the SOA chip as describedabove, only a relationship W_(lens)>W_(TE)>W_(TM) can be achieved evenif the distance of the lens system from the end face of the SOA chip isadjusted, and the relationship η_(TE)>η_(TM) cannot be changed into therelationship η_(TE)<η_(TM). Thus, the inter-polarization difference inoptical coupling loss between the SOA chip and the optical fiber cannotbe added by design.

FIG. 4B illustrates a simulated result of the variation of theinter-polarization difference in optical coupling loss between the SOAchip 3 and the optical fiber 7 when the distance of the lens system 6(the first lens 6A in this embodiment) from the waveguide end face 3X ofthe SOA chip 3 in the optical axis direction is changed. In FIG. 4B, theinter-polarization difference in optical coupling loss is assumed to bea positive value when TE>TM.

In this embodiment, an SOA chip 3 is used that provides spot sizes ofsubstantially 3.0 μm and 4.0 μm on the end face for the TE mode and theTM mode respectively, and a lens system 6 is used that provides a spotsize of substantially 2.0 μm for the condensed beam (signal light) fromthe optical fiber 7 at the beam waist position (the spot sizes arediameters since the spots are substantially circular).

In addition, as illustrated in FIG. 4A, the displacement in the opticalaxis direction is substantially zero when the beam waist position isadjusted to the position of the end face of the SOA chip 3 (that is,when the distance of the lens system 6 from the end face of the SOA chip3 is substantially equal to the lens focal length), the displacement inthe optical axis direction is assumed to be a positive value when thebeam waist position is displaced to the output side from the end face ofthe SOA chip 3 (that is, when the distance of the lens system 6 (thefirst lens 6A in this embodiment) from the end face of the SOA chip 3 islonger than the lens focal length), and the displacement in the opticalaxis direction is assumed to be a negative value when the beam waistposition is displaced to the input side from the end face of the SOAchip 3 (that is, when the distance of the lens system 6 (the first lens6A in this embodiment) from the end face of the SOA chip 3 is shorterthan the lens focal length).

As illustrated in FIG. 4B, if the beam waist position is displaced by ±5μm from the position of the end face of the SOA chip 3 in the opticalaxis direction, a change in inter-polarization difference in opticalcoupling loss of substantially ±1 dB occurs. This means that theinter-polarization difference in optical coupling loss can be adjustedwithin a range of 1 dB in both the directions of TE>TM and TE<TM.

As can be seen from the simulated result, when the SOA chip 3 and thelens system 6 described above are used, the PDG of the SOA chip 3 can becompensated for within a range of substantially ±1 dB, and the SOAmodule 1 having a PDG of substantially 0 can be manufactured if thevariations of the PDG of the SOA chip 3 falls within the above range.

As described above, by adjusting the inter-polarization difference inoptical coupling loss between the SOA chip 3 and the output-side opticalfiber 7, the PDG in the SOA chip can be cancelled, thereby compensatingfor a variation of the PDG of the SOA chip 3 and thus providing a SOAmodule 1 that achieves a low PDG, stably.

In the following, a specific example of the manufacturing method for theSOA module according to the present invention will be described withreference to FIGS. 5 and 6.

As illustrated in FIG. 5A, first, the SOA chip 3 is fixed on the stage(a temperature-controllable stage) 8, electrical wiring to energize theSOA chip 3 is installed, and then, a current is supplied to the SOA chip3 to make the SOA chip 3 emit ASE light (amplified spontaneous emissionlight) from both of the end faces (S10 in FIG. 6).

Then, as illustrated in FIG. 5A, positional adjustment of the opticalsystem is carried out to reduce the optical coupling loss between theSOA chip 3 and the input-side optical fiber 5 and the output-sideoptical fiber 7.

As illustrated in FIG. 5A, the input-side optical fiber 5 and theinput-side lens system 4 are temporarily positioned on the input side ofthe SOA chip 3, and the output-side lens system 6 and the output-sideoptical fiber 7 are temporarily positioned on the output side of the SOAchip 3. Then, the positions of the lens systems 4 and 6 and the opticalfibers 5 and 7 on both the input and the output side are adjusted toincrease the power of the ASE light (the ASE light power) opticallycoupled from the end faces of the SOA chip 3 to the optical fibers 5 and7 via the lens systems 4 and 6 (that is, to reduce the optical couplingloss between the SOA chip 3 and the optical fibers 5 and 7), and theinput-side lens system 4, the input-side optical fiber 5, and theoutput-side optical fiber 7 are fixed (steps S20 and S30 in FIG. 6).

For example, when the input-side lens system 4 and the output-side lenssystem 6 each include two lenses as illustrated in FIG. 5A, thepositions of the input-side lens system 4, the output-side lens system6, the input-side optical fiber 5, and the output-side optical fiber 7are adjusted, and the input-side lens system 4, the input-side opticalfiber 5, and the output-side optical fiber 7 are fixed as describedbelow.

In this embodiment, of the two lenses 6A and 6B in the output-side lenssystem 6, the lens disposed closer to the SOA chip 3 is referred to asthe first lens 6A, and the lens disposed closer to the optical fiber 7is referred to as the second lens 6B. Of the two lenses 4A and 4B in theinput-side lens system 4, the lens disposed closer to the SOA chip 3 isreferred to as the third lens 4A, and the lens disposed closer to theoptical fiber 5 is referred to as the fourth lens 4B.

As illustrated in FIG. 5A, the first lens 6A in the output-side lenssystem 6 is temporarily positioned, and the position of the first lens6A is adjusted so that the ASE light emitted from one end face of theSOA chip 3 is changed into collimated light by passing through the firstlens 6A (S20 in FIG. 6). In S20, the first lens is not fixed.

Similarly, the third lens 4A in the input-side lens system 4 istemporarily positioned, and the position of the third lens 4A isadjusted so that the ASE light emitted from the other end face of theSOA chip 3 is changed into collimated light by passing through the thirdlens 4A. Then, the third lens 4A is fixed (S20 in FIG. 6).

As illustrated in FIG. 5A, the second lens 6B in the output-side lenssystem 6 and the output-side optical fiber 7 are temporarily positioned.The positions of the second lens 6B and the output-side optical fiber 7are adjusted so that the collimated light formed by passing through thefirst lens 6A is optically coupled to the output-side optical fiber 7with high efficiency, and then, the components are fixed (S30 in FIG.6).

As illustrated in FIG. 5A, for example, the optical power meter 9 isconnected to the output-side optical fiber 7, the positions of thesecond lens 6B and the output-side optical fiber 7 are adjusted toincrease the intensity (power) of the ASE light introduced into theoutput-side optical fiber 7 by monitoring the intensity of the ASE lightwith the optical power meter 9, and the second lens 6B and theoutput-side optical fiber 7 are fixed (S30 in FIG. 6).

Similarly, the fourth lens 4B in the input-side lens system 4 and theinput-side optical fiber 5 are temporarily positioned, the positions ofthe fourth lens 4B and the input-side optical fiber 5 are adjusted sothat the collimated light formed by passing through the third lens 4A isoptically coupled to the input-side optical fiber 5 with highefficiency, and the components are fixed (S30 in FIG. 6).

For example, the optical power meter 9 is connected to the input-sideoptical fiber 5, the positions of the fourth lens 4B and the input-sideoptical fiber 5 are adjusted to increase the intensity (power) of theASE light introduced into the input-side optical fiber 5 by monitoringthe intensity of the ASE light with the optical power meter 9, and thefourth lens 4B and the input-side optical fiber 5 are fixed (S30 in FIG.6).

In this way, the SOA chip 3 is optically coupled to the input-sideoptical fiber 5 and the output-side optical fiber 7 via the input-sidelens system 4 and the output-side lens system 6, respectively, in such amanner that the power of the ASE light coupled from the opposite endfaces of the SOA chip 3 to the input-side optical fiber 5 and theoutput-side optical fiber 7 via the input-side lens system 4 and theoutput-side lens system 6, respectively, is increased.

Note that the procedure of adjusting the lens systems 4 and 6 is notlimited to the procedure described above and may vary with the lenssystems used.

If the positional adjustment is carried out to increase the power of theASE light (if the positional adjustment is carried out based on theoptical power), the optical coupling loss between the SOA chip 3 and theoptical fibers 5 and 7 is reduced.

However, since the inter-polarization difference in optical couplingloss between the SOA chip 3 and the optical fibers 5 and 7 assumes aconstant value that is determined by the optical waveguide mode of theSOA chip 3 and the design of the lens systems 4 and 6, the PDG of theSOA module 1 varies because of the variations of the PDG of the SOA chip3.

Thus, in this embodiment, to compensate for variations of the PDG of theSOA chip 3 during assembly of the module to provide a SOA module 1having a low PDG, positional re-adjustment based on the PDG of the SOAchip 3 is additionally carried out as described below.

In this embodiment, after the positional adjustment to increase thepower of the ASE light is carried out as described above, furtherpositional adjustment is carried out by displacing the first lens 6A inthe output-side lens system 6 in the optical axis direction so that theinter-polarization difference in optical coupling loss between the SOAchip 3 and the output-side optical fiber 7 is suitable for compensationof the PDG of the SOA chip 3 (steps S40 and S50 in FIG. 6).

Note that, to enable such positional adjustment, the first lens 6A inthe output-side lens system 6 is not fixed, while the input-side lenssystem 4 (optical system), the input-side optical fiber 5, the secondlens 6B in the output-side lens system 6 (optical system), and theoutput-side optical fiber 7 are fixed by yttrium aluminum garnet (YAG)welding, ultra violet (UV) resin or the like.

The positional adjustment is carried out as follows.

As illustrated in FIG. 5B, the optical power meter 9 is removed from theinput-side optical fiber 5, and a laser light source/polarizationscrambler 10 is connected to the input-side optical fiber 5 (S40 in FIG.6).

Then, polarization-scrambled signal light (including light in opticalwaveguide modes differing in the direction of polarization, by changingthe polarization state of signal light over time) is input from thelaser light source/polarization scrambler 10 to the temporarilyassembled SOA module 1 via the input-side optical fiber 5 (S40 in FIG.6).

The polarization-scrambled signal light is input to the SOA chip 3 viathe input-side optical fiber 5 and the input-side lens system 4,amplified in the SOA chip 3 to which a current is supplied, and outputto the optical power meter 9 connected to the output-side optical fiber7 via the output-side lens system 6 and the output-side optical fiber 7.

As illustrated in FIG. 5B, the optical power meter 9 connected to theoutput-side optical fiber 7 monitors the temporal waveform of theintensity of the signal light amplified in the SOA chip 3 and outputfrom the SOA module 1 (S40 in FIG. 6).

By monitoring the temporal waveform displayed by the optical power meter9, the variations of the output (variations of the signal lightintensity) corresponding to the rotation of the direction ofpolarization by the polarization scrambler can be monitored. Note thatthe peak to peak amplitude of the output waveform displayed by theoptical power meter 9 indicates the PDG of the SOA module 1.

Then, while monitoring the temporal waveform displayed by the opticalpower meter 9, positional adjustment is carried out by displacing thefirst lens 6A in the output-side lens system 6 in the optical axisdirection (that is, by changing the relationship between the beam waistposition and the position of the output end face of the SOA chip 3) sothat the peak to peak amplitude of the temporal waveform approximatelyequals 0 (that is, the PDG substantially equals 0), and then, the firstlens 6A is fixed (S50 in FIG. 6). In summary, the signal light(including light in optical waveguide modes differing in the directionof polarization) output from the SOA chip 3 via the output-side lenssystem 6 is detected, and the first lens 6A is repositioned at alocation where the PDG is reduced based on the detected light.

By such positional adjustment, the variations of the PDG of the SOA chip3 can be compensated for based on the inter-polarization difference inoptical coupling loss between the SOA chip 3 and the optical fiber 7.

Then, sealing of the module or the like is carried out to complete theSOA module 1 according to this embodiment (S60 in FIG. 6).

The SOA module 1 manufactured as described above has the configurationdescribed below.

As illustrated in FIG. 1, the SOA module 1 according to this embodimenthas the SOA chip 3 that differs in spot size on the chip end face(element end face) between optical waveguide modes differing in thedirection of polarization, and having a PDG in optical waveguide modesdiffering in the direction of polarization. The optical fibers 5 and 7are disposed at the opposite end faces of the SOA chip 3. In addition,the lens systems 4 and 6 are disposed at the opposite end faces of theSOA chip 3. The lens systems 4 and 6 optically couple the SOA chip 3 tothe optical fibers 5 and 7, respectively.

Thus, the signal light introduced through the input-side optical fiber 5is optically coupled to one end face of the optical waveguide of the SOAchip 3 by the input-side lens system 4, amplified in the SOA chip 3,output from the other end face of the optical waveguide of the SOA chip3, and then optically coupled to the output-side optical fiber 7 by theoutput-side lens system 6.

In the SOA module 1 according to this embodiment, the output-side lenssystem 6 is configured to provide a spot size at the beam waist positionsmaller than the spot size in any of the optical waveguide modes.

In addition, the output-side lens system 6 is positioned so that theinter-polarization difference in optical coupling loss is suitable forcompensation for the polarization dependent gain of the SOA chip 3. Thatis, the output-side lens system 6 is positioned at a location where theoutput-side lens system 6 may be displaced in the optical axis directionfrom the location where the distance between the output-side lens system6 and the end face of the SOA chip 3 is equal to the focal length of theoutput-side lens system 6.

In this way, the output-side lens system 6 is positioned so that thespot size of the beam condensed by the output-side lens system 6 on theend face of the SOA chip 3 is smaller than the spot size of both of theoptical waveguide modes or smaller than the spot size in either of theoptical waveguide modes.

In addition, as illustrated in FIG. 1, in the SOA module 1 according tothis embodiment, the SOA chip 3 is fixed on the temperature-controllablestage 8 that is capable of adjusting the temperature of the SOA chip 3(including, for example, a thermistor and a thermoelectric cooler(TEC)).

Therefore, the optical semiconductor device (optical module) and themanufacturing method therefor according to this embodiment have anadvantage that a stable polarization-independent optical gain can beachieved even when the PDG of the SOA chip 3 varies.

That is, according to the configuration of the optical semiconductordevice and the manufacturing method therefor according to thisembodiment, in the SOA module 1 including the SOA chip 3 that providesdifferent shapes of guided light on the element end face for twoorthogonal polarization states (the TE mode and the TM mode) and hassome PDG, the PDG of the SOA chip 3 can be compensated for by adding aninter-polarization difference in optical coupling loss by positionaladjustments of the lenses during manufacture of the module. Therefore, asmall-size SOA module 1 having a stable and low PDG may be provided witha simple configuration without an additional optical component ormechanism.

Second Embodiment

Next, an optical semiconductor device and a manufacturing methodtherefor according to a second embodiment will be described withreference to FIGS. 7 and 8.

The optical semiconductor device (optical module; SOA module) and themanufacturing method therefor according to this embodiment differ fromthose according to the first embodiment described above, in which theinter-polarization difference in optical coupling loss is added only tothe output-side, in that the inter-polarization difference in opticalcoupling loss is added to both the input side and the output side.

In the first embodiment described above, the PDG of the SOA chip 3 iscompensated for by adding the inter-polarization difference in opticalcoupling loss only to the output-side of the SOA module 1. As a result,the SOA module 1 manufactured by the manufacturing method according tothe first embodiment described above may have different characteristicsdepending on the direction of the input light.

However, in some applications of the SOA module 1, it may be preferablefor the SOA module 1 to have constant characteristics independent of thedirection of the input light. In such a case, the PDG of the SOA chip 3can be compensated for by adding the inter-polarization difference inoptical coupling loss both to the input side and the output side of theSOA module 1.

To achieve this, according to the manufacturing method for the SOAmodule according to this embodiment, an SOA module 1 is manufactured asfollows.

First, an SOA chip (semiconductor device; optical device) 3, aninput-side lens system (one lens system) 40, an output-side lens system(the other lens system) 6, an input-side optical fiber 5 and anoutput-side optical fiber 7 (one optical fiber and the other opticalfiber) are provided (see FIG. 7A). Note that, in FIG. 7A, the samecomponents as those in the first embodiment described above are denotedby the same reference numerals as those in the first embodiment.

Lens systems that provide spot sizes at a beam waist position smallerthan the spot sizes on the end face of the SOA chip 3 in any of theoptical waveguide modes (the TE mode and the TM mode in this embodiment)are provided as the input-side lens system 40 and the output-side lenssystem 6.

In this embodiment, the output-side lens system 6 includes a first lens6A and a second lens 6B (see FIG. 7A). The input-side lens system 40includes a third lens 40A and a fourth lens 40B (see FIG. 7A).

Then, the input-side lens system 40 and the input-side optical fiber 5are disposed on the side of the input end face (one end face) of the SOAchip 3, and the output-side lens system 6 and the output-side opticalfiber 7 are disposed on the output-side end face (the other end face) ofthe SOA chip 3 (see FIG. 7A).

In this embodiment, on the side of the input end face of the SOA chip 3,the third lens 40A is disposed next to the SOA chip 3, the fourth lens40B is disposed next to the third lens 40A on the side away from the SOAchip 3, and the input-side optical fiber 5 is disposed next to thefourth lens 40B furthest away from the SOA chip 3 (see FIG. 7A).

On the side of the output end face of the SOA chip 3, the first lens 6Ais disposed next to the SOA chip 3, the second lens 6B is disposed nextto the first lens 6A on the side away from the SOA chip 3, and theoutput-side optical fiber 7 is disposed next to the second lens 6Bfurthest away from the SOA chip 3 (see FIG. 7A).

As in the first embodiment described above (see FIG. 2), the positionsof the input-side lens system 40, the output-side lens system 6, theinput-side optical fiber 5 and the output-side optical fiber 7 areadjusted to increase the power of the light coupled from the end facesof the SOA chip 3 to the input-side optical fiber 5 and the output-sideoptical fiber 7 via the input-side lens system 40 and the output-sidelens system 6, respectively (that is, to reduce the optical couplingloss).

In this embodiment, as in the first embodiment described above (see FIG.2), by applying a current to the SOA chip 3 to make the SOA chip 3 emitASE light from the end faces, the positions of the first lens 6A, thesecond lens 6B, and the output-side optical fiber 7 are adjusted toincrease the power of the light coupled to the output-side optical fiber7 via the first lens 6A and the second lens 6B in the output-side lenssystem 6.

Similarly, the positions of the third lens 40A, the fourth lens 40B andthe input-side optical fiber 5 are adjusted to increase the power of thelight coupled to the input-side optical fiber 5 via the third lens 40Aand the fourth lens 40B in the input-side lens system 40.

After the positions are adjusted, the fourth lens 40B, the input-sideoptical fiber 5, the second lens 6B, and the optical-side optical fiber7 are fixed.

In this embodiment, after the positions are adjusted based on theoptical power as described above, positional adjustment is carried outby displacing the input-side lens system 40 (the third lens 40A in thisembodiment) and the output-side lens system 6 (the first lens 6A in thisembodiment) in the optical axis direction so that the inter-polarizationdifference in optical coupling loss between the SOA chip 3 and theinput-side optical fiber 5 and the inter-polarization difference inoptical coupling loss between the SOA chip 3 and the output-side opticalfiber 7 are suitable for compensation for the PDG of the SOA chip 3 (seeFIG. 3A).

That is, in this embodiment, in the optical coupling adjustment process(lens system adjustment process) during manufacture of the SOA module,the third lens 40A and the first lens 6A that have been adjusted inposition to reduce the optical coupling loss as described above areintentionally shifted in the optical axis direction (see FIG. 3A).

For example, even if the focal position of the output-side lens system 6has been adjusted to the output end face of the SOA chip 3 by the firstpositional adjustment of the output-side lens system 6 described above(see the position depicted by the solid line in FIG. 3A), the focalposition of the output-side lens system 6 is shifted from the positionof the output end face of the SOA chip 3 by the second positionaladjustment of the output-side lens system 6 (see the position depictedby the broken line in FIG. 3A). Similarly, even if the focal position ofthe input-side lens system 40 has been adjusted to the input end face ofthe SOA chip 3 by the first positional adjustment of the input-side lenssystem 40 described above, the focal position of the input-side lenssystem 40 is shifted from the position of the input end face of the SOAchip 3 by the second positional adjustment of the input-side lens system40.

As a result, an optical coupling loss may occur in the SOA module 1.However, the inter-polarization difference in optical coupling lossbetween the SOA chip 3 and the input-side optical fiber 5 and theinter-polarization difference in optical coupling loss between the SOAchip 3 and the output-side optical fiber 7 can be adjusted.

After the positional adjustment based on the PDG of the SOA chip 3 iscarried out, the third lens 40A and the first lens 6A are fixed.

In this way, a polarization-independent SOA module that provides apolarization-independent optical gain, which is one of the opticalamplification characteristics of the part of the module 1 between theinput-side optical fiber 5 and the output-side optical fiber 7, can bemanufactured.

In the following, an example of the manufacturing method for the SOAmodule according to the present invention will be described withreference to FIGS. 7A to 7C and 8.

First, as in the first embodiment described above, as illustrated inFIG. 7A, the SOA chip 3 is fixed on a stage (a temperature-controllablestage) 8, and electrical wiring to energize the SOA chip 3 is installed.A current is supplied to the SOA chip 3 to make the SOA chip 3 emit ASElight (amplified spontaneous emission light) from the opposite end faces(A10 in FIG. 8).

Then, as illustrated in FIG. 7A, positional adjustment of the opticalsystem is carried out to reduce the optical coupling loss between theSOA chip 3 and the input-side optical fiber 5 and the output-sideoptical fiber 7.

First, as illustrated in FIG. 7A, the first lens 6A in the output-sidelens system 6 is temporarily positioned, and the position of the firstlens 6A is adjusted so that the ASE light emitted from one end face ofthe SOA chip 3 is changed into collimated light by passing through thefirst lens 6A (A20 in FIG. 8). In A20, the first lens 6A is not fixed.

Similarly, as illustrated in FIG. 7A, the third lens 40A in theinput-side lens system 40 is temporarily positioned, and the position ofthe third lens 40A is adjusted so that the ASE light emitted from theother end face of the SOA chip 3 is changed into collimated light bypassing through the third lens 40A (A20 in FIG. 8). In the A20, thethird lens 40A is not fixed.

Then, as illustrated in FIG. 7A, the second lens 6B in the output-sidelens system 6 and the output-side optical fiber 7 are temporarilypositioned, the positions of the second lens 6B and the output-sideoptical fiber 7 are adjusted so that the collimated light formed bypassing through the first lens 6A is optically coupled to theoutput-side optical fiber 7 with high efficiency, and then, thesecomponents are fixed (A30 in FIG. 8).

As illustrated in FIG. 7A, for example, the optical power meter 9 isconnected to the output-side optical fiber 7, the positions of thesecond lens 6B and the output-side optical fiber 7 are adjusted toincrease the intensity (power) of the ASE light introduced into theoutput-side optical fiber 7 by monitoring the intensity of the ASE lightwith the optical power meter 9, and then, the second lens 6B and theoutput-side optical fiber 7 are fixed (A30 in FIG. 8).

Similarly, as illustrated in FIG. 7A, the fourth lens 40B in theinput-side lens system 40 and the input-side optical fiber 5 aretemporarily positioned, the positions of the fourth lens 40B and theinput-side optical fiber 5 are adjusted so that the collimated lightformed by passing through the third lens 40A is optically coupled to theinput-side optical fiber 5 with high efficiency, and then, thesecomponents are fixed (A30 in FIG. 8).

As illustrated in FIG. 7A, for example, the optical power meter 9 isconnected to the input-side optical fiber 5, the positions of the fourthlens 40B and the input-side optical fiber 5 are adjusted to increase theintensity (power) of the ASE light introduced into the input-sideoptical fiber 5 by monitoring the intensity of the ASE light with theoptical power meter 9, and then, the fourth lens 40B and the input-sideoptical fiber 5 are fixed (A30 in FIG. 8).

In this way, the SOA chip 3 is optically coupled to the input-sideoptical fiber 5 and the output-side optical fiber 7 via the input-sidelens system 40 and the output-side lens system 6, respectively, in sucha manner that the power of the ASE light coupled from the opposite endfaces of the SOA chip 3 to the input-side optical fiber 5 and theoutput-side optical fiber 7 via the input-side lens system 40 and theoutput-side lens system 6, respectively, is increased.

Note that the procedure of adjusting the lens systems 40 and 6 is notlimited to the procedure described above and may vary with the lenssystems used.

After the positional adjustment to increase the power of the ASE light(positional adjustment based on the optical power) is carried out asdescribed above, further positional adjustment based on the PDG of theSOA chip 3 is carried out as described below.

Positional adjustment is carried out by displacing the third lens 40A inthe input-side lens system 40 and the first lens 6A in the output-sidelens system 6 in the optical axis direction so that theinter-polarization difference in optical coupling loss between the SOAchip 3 and the input-side optical fiber 5 and the inter-polarizationdifference in optical coupling loss between the SOA chip 3 and theoutput-side optical fiber 7 are suitable for compensation for the PDG ofthe SOA chip 3 (steps A40 and A50 in FIG. 8).

Note that, to enable such positional adjustment, the third lens 40A inthe input-side lens system 40 and the first lens 6A in the output-sidelens system 6 are not fixed, while the fourth lens 40B in the input-sidelens system 40, the input-side optical fiber 5, the second lens 6B inthe output-side lens system 6, and the output-side optical fiber 7 arefixed by YAG welding, UV resin or the like, as described above.

Specifically, the positional adjustment is carried out as follows.

As illustrated in FIG. 7B, the optical power meter 9 is removed from theinput-side optical fiber 5, and a laser light source/polarizationscrambler 10 is connected to the input-side optical fiber 5, for example(A40 in FIG. 8).

Then, polarization-scrambled signal light, which includes light inoptical waveguide modes differing in the direction of polarization bychanging the polarization state of the signal light over time, is inputfrom the laser light source/polarization scrambler 10 to the temporarilyassembled SOA module 1 via the input-side optical fiber 5 (A40 in FIG.8).

The polarization-scrambled signal light is input to the SOA chip 3 viathe input-side optical fiber 5 and the input-side lens system 40,amplified in the SOA chip 3 to which a current is supplied, and thenoutput to the optical power meter 9 connected to the output-side opticalfiber 7 via the output-side lens system 6 and the output-side opticalfiber 7.

As illustrated in FIG. 7B, the optical power meter 9 connected to theoutput-side optical fiber 7 monitors the temporal waveform of theintensity of the signal light amplified in the SOA chip 3 and outputfrom the SOA module 1 (the power of the output signal light) (A40 inFIG. 8).

Then, while monitoring the temporal waveform displayed by the opticalpower meter 9, positional adjustment is carried out by displacing thefirst lens 6A in the output-side lens system 6 in the optical axisdirection (that is, by changing the relationship between the beam waistposition and the position of the output end face of the SOA chip 3) sothat the peak to peak amplitude of the temporal waveform substantiallyequals half of an initial value, for example, and then, the first lens6A is fixed (A40 in FIG. 8).

Then, to reverse the direction of inputting light and introduce thesignal light from the output side, as illustrated in FIG. 7C, the laserlight source/polarization scrambler 10 connected to the input-sideoptical fiber 5 is removed, and the optical power meter 9 is connectedto the input-side optical fiber 5. In addition, the optical power meter9 connected to the output-side optical fiber 7 is removed, and the laserlight source/polarization scrambler 10 is connected to the output-sideoptical fiber 7 (A50 in FIG. 8).

Then, polarization-scrambled signal light (including light in opticalwaveguide modes differing in the direction of polarization) is inputfrom the laser light source/polarization scrambler 10 to the temporarilyassembled SOA module 1 via the output-side optical fiber 7 (A50 in FIG.8).

The polarization-scrambled signal light is input to the SOA chip 3 viathe output-side optical fiber 7 and the output-side lens system 6,amplified in the SOA chip 3 to which a current is supplied, and thenoutput to the optical power meter 9 connected to the input-side opticalfiber 5 via the input-side lens system 40 and the input-side opticalfiber 5.

As illustrated in FIG. 7C, the optical power meter 9 connected to theinput-side optical fiber 5 monitors the temporal waveform of theintensity of the signal light amplified in the SOA chip 3 and outputfrom the SOA module 1 (the power of the output signal light) (A50 inFIG. 8).

Then, while monitoring the temporal waveform displayed by the opticalpower meter 9, positional adjustment is carried out by displacing thethird lens 40A in the input-side lens system 40 in the optical axisdirection (by changing the relationship between the beam waist positionand the position of the input end face of the SOA chip 3) so that theamplitude of the temporal waveform approximately equals 0 (that is, thePDG is substantially 0), and then, the third lens 40A is fixed (A50 inFIG. 8). In summary, the signal light (including light in opticalwaveguide modes differing in the direction of polarization) output fromthe SOA chip 3 via the input-side lens system 40 is detected, and thethird lens 40A is repositioned at a location where the PDG is reducedbased on the detected light.

By such positional adjustment, the variations of the PDG of the SOA chip3 can be compensated for based on the inter-polarization difference inoptical coupling loss added equally to the input side and the outputside (the inter-polarization difference in optical coupling loss betweenthe SOA chip 3 and the input-side optical fiber 5 and theinter-polarization difference in optical coupling loss between the SOAchip 3 and the output-side optical fiber 7).

Then, sealing of the module or the like is carried out to complete theSOA module 1 according to this embodiment (A60 in FIG. 8).

The remaining details are the same as those in the first embodimentdescribed earlier, and therefore, descriptions thereof will be omitted.

The SOA module 1 manufactured as described above has the configurationdescribed below.

In the SOA module 1 according to this embodiment, both the input-sidelens system 40 and the output-side lens system are 6 are configured toprovide a spot size at the beam waist position smaller than the spotsize in any of the optical waveguide modes.

In addition, both the input-side lens system 40 and the output-side lenssystem 6 are positioned so that the inter-polarization difference inoptical coupling loss is suitable for compensation for the polarizationdependent gain of the SOA chip 3. That is, the lens systems 40 and 6 areeach positioned at a location where the lens systems 40 and 6 aredisplaced in the optical axis direction from the location where thedistance between the lens systems 40 and 6 and the end face of the SOAchip 3 is equal to the focal length of the lens systems 40 and 6.

In particular, in this embodiment, an equal inter-polarizationdifference in optical coupling loss is added to the input side and theoutput side of the SOA module 1.

In this way, both the lens systems 40 and 6 are positioned so that thespot sizes of the beams condensed by the lens systems 40 and 6 on theend faces of the SOA chip 3 are smaller than the spot sizes in both ofthe optical waveguide modes or smaller than the spot size in either ofthe optical waveguide modes.

The remaining details are the same as those in the first embodimentdescribed earlier, and therefore, descriptions thereof will be omitted.

In summary, the manufacturing method for an optical semiconductor device(optical module) and the optical module according to this embodimenthave an advantage that a stable polarization-independent optical gaincan be achieved even when the PDG of the SOA chip 3 varies.

In particular, this embodiment has an advantage that the SOA module 1whose characteristics do not vary with the direction of the input lightis provided since the inter-polarization difference in optical couplingloss is added substantially equally to the input side and the outputside of the SOA module 1.

In the embodiment described above, positional adjustment is carried outso that the amplitude of the temporal waveform substantially equals halfof the initial value by displacing the first lens 6A in the output-sidelens system 6 only in the optical axis direction, and then, furtherpositional adjustment is carried out so that the amplitude of thetemporal waveform approximately equals 0 by displacing the third lens40A in the input-side lens system 40 only in the optical axis direction.However, the present invention is not limited thereto, and only thevariations of the PDG of the SOA chip 3 may be compensated for by theinter-polarization difference in optical coupling loss added to both theinput side and the output side of the SOA module 1. For example,positional adjustment may be carried out so that the amplitude of thetemporal waveform approximately equals a specific value by displacingthe first lens 6A in the output-side lens system 6 only in the opticalaxis direction, and then, further positional adjustment may be carriedout so that the amplitude of the temporal waveform approximately equals0 by displacing the third lens 40A in the input-side lens system 40 onlyin the optical axis direction.

In the manufacturing methods for the SOA module according to theembodiments described above, the output-side lens system 6 in the firstembodiment and the input-side lens system 40 and the output-side lenssystem 6 in the second embodiment are configured to provide a spot sizeat the beam waist position smaller than the spot size in any of the TEmode and the TM mode. However, the present invention is not limitedthereto.

For example, in the first embodiment, if the SOA chip 3 can providesignificantly different spot sizes on the chip end face in the TE modeand the TM mode, and the PDG of the SOA chip 3 can be compensated for bythe inter-polarization difference in optical coupling loss between theSOA chip 3 and the optical fiber 7, the output-side lens system 6 may bea lens system configured to provide a spot size at the beam waistposition smaller than the spot size in either of the TE mode and the TMmode.

In addition, for example, in the second embodiment, if the SOA chip 3can provide significantly different spot sizes on the chip end face inthe TE mode and the TM mode, and the PDG of the SOA chip 3 can becompensated for by the inter-polarization difference in optical couplingloss between the SOA chip 3 and the optical fiber 5 and theinter-polarization difference in optical coupling loss between the SOAchip 3 and the optical fiber 7, the input-side lens system 40 and theoutput-side lens system 6 may be lens systems configured to provide aspot size at the beam waist position smaller than the spot size ineither of the TE mode and the TM mode.

In such a case, the SOA module manufactured by the manufacturing methodaccording to the embodiments described above is configured so that theoutput-side lens system 6 in the first embodiment (or the input-sidelens system 40 and the output-side lens system 6 in the secondembodiment) provides a spot size at the beam waist position smaller thanthe spot size in either of the optical waveguide modes.

In addition, in the embodiments described above, the lens disposed at aposition closest to the end face of the SOA chip 3 (the first lens 6A inthe first embodiment, and the first lens 6A and the third lens 40A inthe second embodiment) is not fixed when the first lens positionadjustment carried out to increase the intensity of the ASE light and isdisplaced only in the optical axis direction when the second lensposition adjustment for adjusting the PDG is carried out.

In this case, to prevent the lens 6A (40A) from moving in two directionsorthogonal to the optical axis (directions in the vertical plane) whenthe lens 6A (40A) is displaced in the optical axis direction, separatefixing mechanisms for the optical axis direction and the two orthogonaldirections are preferably provided.

For example, a lens jig capable of moving the lens 6A (40A) in theoptical axis direction and fixing the lens 6A (40A) in the twodirections orthogonal to the optical axis direction can be used.

For example, as illustrated in FIGS. 9A and 9B, a lens jig 15 having afirst guide member 11 capable of guiding the lens 6A (40A) in thevertical direction orthogonal to the optical axis direction and a secondguide member 12 capable of guiding the first guide member 11 in theoptical axis direction can be used. In this case, the lens 6A (40A) isfixed on a lens holder 13. In addition, the second guide member 12 isfixed on the stage 8 (see FIG. 1).

Using a lens jig 15, the lens 6A (40A) is preferably fixed after thefirst lens position adjustment in such a manner that the lens 6A (40A)can move in the optical axis direction but cannot move in the twodirections orthogonal to the optical axis direction.

That is, when the lens holder 13 to which the lens 6A (40A) is fixed ismoved in the vertical direction orthogonal to the optical axis directionin the first lens position adjustment, the lens holder 13 to which thelens 6A (40A) is fixed can be adjusted in the orthogonal directions tothe optical axis using the first guide member 11. After the lensposition adjustment in the vertical direction orthogonal to the opticalaxis direction, the inner surface of the side wall of the first guidemember 11 and the side surface of the lens holder 13 are fixed to eachother by welding, resin or the like as depicted by reference character Xin FIG. 9B.

In this way, in the second lens position adjustment for adjusting thePDG, an accidental displacement of the lens in the two directionsorthogonal to the optical axis direction can be prevented, so that theSOA module 1 manufactured can have a further reduced coupling loss.

In FIGS. 9A and 9B, for clarity of explanation, clearances areillustrated between the side surface of the lens holder 13 and the innersurface of the side wall of the first guide member 11 and between theouter surface of the side wall of the first guide member 11 and theinner surface of the side wall of the second guide member 12. However,the actual clearance may be very small. Therefore, lens positionadjustment in the orthogonal plane direction to the optical axis can beachieved by fixing the second guide member 12 on the stage 8 (see FIG.1), mounting the first guide member 11 on the second guide member 12,and mounting the lens holder 13 on the first guide member 11.

Then, when the lens holder 13 to which the lens 6A (40A) is fixed ismoved in the optical axis direction in the lens position adjustment foradjusting the PDG, the second guide member 12 guides the first guidemember 11 to which the lens holder 13 is fixed, only in the optical axisdirection. Note that, in advance, the second guide member 12 is fixed tothe stage 8 in a way that the direction of the side wall of the secondguide member 12 (horizontal side wall of 12 in FIG. 9A) is adjusted tobe parallel to the optical axis. Then, after the lens positionadjustment in the optical axis direction, the inner surface of the sidewall of the second guide member 12 and the outer surface of the sidewall of the first guide member 11 are fixed to each other by welding,resin or the like as depicted by reference character Y in FIG. 9B.

The SOA module 1 manufactured using the lens jig 15 includes the firstguide member 11 and the second guide member 12. The lens 6A (40A)disposed at a position closest to the end face of the SOA chip 3 isfixed to the first guide member 11, and the first guide member 11 isfixed to the second guide member 12.

In addition, the configuration of the SOA module 1 for each embodimentdescribed above can be varied.

For example, the spot size provided by the SOA chip 3 and the spot sizesprovided at the beam waist position by the lens systems 40 and 6 in theembodiments described above are not limited to the values described inthe embodiments and may have other values.

In addition, the manufacturing procedure and configuration of the module1 are not limited to those in the embodiments described above.

In addition, in the embodiments described above, as illustrated in FIG.10, the SOA module 1 may have an SOA chip (a semiconductor device, anintegrated device, an optical device) 30 integrated with a spot sizetransformer 16 in the vicinity of an end face thereof. Note that FIG. 10illustrates only the output-side part of the SOA chip including theoutput-side end face.

In this example, as illustrated in FIG. 10, the spot size transformer 16is an optical waveguide formed to be seamlessly connected to an opticalwaveguide (SOA optical waveguide) 30B forming an SOA part 30A of the SOAchip 30, and the optical waveguide is a tapered optical waveguide whosewidth is gradually reduced toward the chip end face (element end face)(a tapered width spot size transformer).

Note that the configuration of the spot size transformer 16 is notlimited to the configuration described above. For example, the spot sizetransformer 16 may be a tapered optical waveguide whose thickness isgradually reduced (a tapered thickness spot size transformer).

By using such a SOA chip 30, the spot size on the end face of the SOAchip 30 in the TE mode and the TM mode can be adjusted by the design ofthe spot size transformer independently of the configuration of the SOAoptical waveguide 30B.

As a result, the SOA chip 30 whose PDG can be easily adjusted duringassembly of the module without degrading the characteristics of the SOAchip 30 (that is, the SOA chip 30 that can provide significantlydifferent spot sizes in the TE mode and the TM mode) can bemanufactured, and the SOA module 1 having preferable characteristics canbe manufactured.

Note that the spot size transformer 16 may be provided in the vicinityof only one of the end faces of the SOA chip 30 or in the vicinity ofthe both end faces thereof.

In addition, in the configurations according to the embodimentsdescribed above, the SOA module 1 may have one lens on the input sideinstead of having the input-side lens system. In addition, in theconfigurations according to the embodiments described above, the SOAmodule 1 may have one lens on the output side instead of having theoutput-side lens system.

In addition, in the configurations according to the embodimentsdescribed above, as illustrated in FIG. 11, the SOA module 1 may have aninput-side lens fiber 20 that is processed to have a lens-like tip endon the input side instead of having the input-side lens system and theinput-side optical fiber and be held by an input-side fiber holder (aconnector) 21. Note that, in FIG. 11, the same components as those inthe embodiments described above (see FIG. 1) are denoted by the samereference numerals.

In addition, in the configurations according to the embodimentsdescribed above, as illustrated in FIG. 11, the SOA module 1 may have anoutput-side lens fiber 22 on the output side that is processed to have alens-like tip end instead of the output-side lens system and theoutput-side optical fiber, and to be held by an output-side fiber holder(a connector) 23.

As described above, the lenses of the SOA module 1 may be formed on anend face of an optical fiber.

In such a case, the spot sizes provided at the beam waist position bythe lens fiber 20 and 22 may be changed by changing the radius ofcurvature of the lens formed on the tip end of the lens fiber.

In addition, in the configurations according to the embodimentsdescribed above, as illustrated in FIG. 12, an optical isolator 24 maybe interposed between the first lens 6A and the second lens 6B to reducethe reflected light. Similarly, as illustrated in FIG. 12, an opticalisolator 25 may be interposed between the third lens 4A (40A) and thefourth lens 4B (40B) to reduce the reflected light. The opticalisolators 24 and 25 are preferably disposed between the SOA chip 3 andthe optical fiber 7 and between the SOA chip 3 and the optical fiber 5.

In addition, in the embodiments described above, the SOA module 1 hasbeen described as having the optical fibers 5 and 7. However, thepresent invention is not limited thereto. For example, the SOA module 1may have a holder (connector) to which an optical fiber is connected. Insuch a case, in the manufacturing methods described above, in adjustingthe position of the optical fiber, the position of the connector of theSOA module (optical module) can be adjusted.

In addition, in the embodiments described above, the present inventionmay be applied to an SOA module using a transmission-type SOA chip asthe SOA chip 3, for example. However, the present invention is notlimited thereto. For example, the present invention can equally beapplied to the SOA module 1 using a reflection-type SOA chip 62 asillustrated in FIG. 13. Note that, in FIG. 13, the same components asthose in the embodiments described above (see FIG. 1) are denoted by thesame reference numerals.

The transmission-type SOA chip 3 in the embodiments described above hasan anti reflection (AR) coating on both the end faces thereof. However,as illustrated in FIG. 13, the reflection-type SOA chip 62 may have ahigh reflecting (HR) coating on one end face, although thereflection-type SOA chip 62 may not have an anti-reflection (AR) coatingon the other end face as with the transmission-type SOA chip 3.

In the case of such a reflection-type SOA chip 62, signal light is inputto the end face with the AR coating, amplified while propagating throughthe device in one direction, reflected from the end face with the HRcoating, amplified while propagating through the device in the oppositedirection, and then output from the end face with the AR coating.

Thus, the SOA module 1 with the reflection-type SOA chip 62 has aninput/output lens system 60 and an input/output optical fiber 61 only onthe side of the end face with the AR coating. In this example, the lenssystem 60 includes a first lens 60A and a second lens 60B.

In the case of such an SOA module 1, signal light input to theinput/output optical fiber 61 is amplified and reflected in thereflection-type SOA chip 62, and output from the same input/outputoptical fiber 61 in the opposite direction.

In this case, an SOA chip that provides different spot sizes on the endface with the AR coating for optical waveguide modes differing in thedirection of polarization is prepared as the reflection-type SOA chip62. The SOA chip has different gains in optical waveguide modesdiffering in the direction of polarization.

In addition, a lens system that provides a spot size at the beam waistposition smaller than the spot size on the end face of thereflection-type SOA chip 62 in each of the optical waveguide modes isprepared as the input/output lens system 60.

On the side of the end face with the AR coating of the reflection-typeSOA chip 62, the first lens 60A is disposed next to the reflection-typeSOA chip 62, the second lens 60B is disposed next to the first lens 60A,and the input/output optical fiber 61 is disposed next to the secondlens 60B.

Then, after the positions of the input/output lens system 60 and theinput/output optical fiber 61 are adjusted to reduce the opticalcoupling loss, further positional adjustment can be carried out based onthe PDG of the reflection-type SOA chip 62 to correct the PDG of thereflection-type SOA chip 62. In this way, the SOA module 1 that has aconstantly low PDG can be manufactured.

In addition, in the embodiments described above, the SOA module 1 havingthe SOA chip 3 has been taken as an example. However, the presentinvention is not limited thereto and can be applied to a wide variety ofoptical semiconductor devices (optical modules) that have asemiconductor element (optical element) that provides different spotsizes on the end face for optical waveguide modes differing in thedirection of polarization and slightly different gains or losses inoptical waveguide modes differing in the direction of polarization. Byapplication of the present invention, the PDG or PDL of thesemiconductor element can be compensated for, and stablepolarization-independent characteristics can be achieved even when thePDG or PDL of the semiconductor element varies.

For example, as illustrated in FIG. 14, the present invention can beapplied to an optical module (e.g., a semiconductor optical integratedelement module, an optical semiconductor device) that has an integratedelement (e.g., an integrated semiconductor optical function element, asemiconductor element, an optical element) including an SOA 31corresponding to the SOA chip 3 in the embodiments described above, andan optical function element 32 (e.g., an electroabsorption (EA) opticalmodulator, in this example) such as a laser light source or an opticalmodulator, which are integrated on the same substrate (a singlesubstrate), as the optical semiconductor device (optical module) havinga semiconductor element that provides different spot sizes on the endface for optical waveguide modes differing in the direction ofpolarization, and slightly different gains or losses in opticalwaveguide modes differing in the direction of polarization. In thiscase, the PDG or PDL of the integrated element 33 can be compensatedfor, and stable polarization-independent characteristics can be achievedeven when the PDG or PDL of the semiconductor element varies.

For example, in the case of the integrated element comprising the SOA 31corresponding to the SOA chip 3 in each embodiment described above andthe optical function element 32 (e.g., EA modulator) integrated on thesame substrate illustrated in FIG. 14, a current (e.g., an SOA current,DC) is supplied to the SOA 31 via an electrode 31A, and a modulationsignal is supplied to the optical function element 32 (e.g., EAmodulator) via an electrode 32A. Continuous light (CW) input to oneelement end face is amplified by the SOA 31, modulated by the opticalfunction element 32 (e.g., EA modulator) and then output from the otherelement end face in the form of a modulated signal (signal light).

In addition, the present invention may also be applied to an opticalsemiconductor device that has an N:1 SOA gate switch element thatincludes a plurality of SOAs and an N:1 optical coupler integrated witheach other, a wavelength switch element comprising a combination of aplurality of SOAs and array waveguide gratings (AWG), or various opticalsignal processing elements comprising a combination of an SOA, a laserlight source and a Mach-Zehnder interferometer, for example.

In addition, as illustrated in FIG. 15, for example, the SOA module(optical module) 1 according to each embodiment described above can beused to form an optical receiver 50 or an optical communication system51.

For example, as illustrated in FIG. 15, the SOA module 1 according toeach embodiment described above can be combined with an optical bandpassfilter 52, an optical detector 53 and the like to form the opticalreceiver 50. In this example, the SOA module 1 is placed in front of theoptical detector 53 in the optical receiver 50. The SOA module 1 servesas a preamplifier that amplifies the optical signal having beenattenuated in an optical transmission path 54 (e.g., an optical fibertransmission path in this example) and forwards the amplified opticalsignal to the optical detector 53. In this case, the SOA module 1according to the present invention having a low PDG is particularlyuseful because the polarization state of the signal light which istransmitted through the optical fiber becomes random and changes overtime.

Furthermore, as illustrated in FIG. 15, the optical receiver 50 thusconfigured can be combined with an optical transmitter 55 to form theoptical communication system 51. That is, the optical communicationsystem 51 comprising the optical receiver 50 described above and theoptical transmitter 55 connected thereto by the optical transmissionpath 54 (e.g., optical fiber transmission path in this example) can beprovided.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical semiconductor device, comprising: asemiconductor element that provides different spot sizes on an elementend face for optical waveguide modes differing in the direction ofpolarization; and a lens positioned at one end face side of thesemiconductor element, wherein a spot size at a beam waist position ofthe lens is smaller than a spot size in either of the optical waveguidemodes.
 2. The optical semiconductor device according to claim 1, whereinthe spot size at a beam waist position of the lens is smaller than thespot size in any of the optical waveguide modes.
 3. The opticalsemiconductor device according to claim 1, further comprising: anotherlens positioned at the other end face side of the semiconductor element.4. The optical semiconductor device according to claim 3, wherein a spotsize at a beam waist position of the another lens is smaller than thespot size in either of the optical waveguide modes.
 5. The opticalsemiconductor device according to claim 4, wherein the spot size at abeam waist position of the another lens is smaller than the spot size inany of the optical waveguide modes.